In order to provide a communications network with the capability to accommodate current and future high bandwidth (broadband) services, optical fiber is being extended deeper into the network, towards the end user. The final link to homes or businesses in present-day systems is often still part of the installed distribution infrastructure, comprised mainly of twisted pairs of copper wire arranged in a topology of distribution cables and drop lines. For high-bandwidth applications, signal loss along a twisted pair increases with frequency and so the length of the twisted pairs must be kept small, leading to deeper penetration of the fiber.
In fact, it is known that the loss in decibels is nonlinearly related to the frequency of measurement (raised to the power 0.5 to 0.7, depending on the frequency and the type of cable) and hence a cable with a loss of, for example, 20 dB at 1 MHZ would have a loss of at least 28 dB at 2 MHZ, and at least 40 dB at 4 MHZ. Moreover, the signal loss in a twisted pair is also proportional to its length. It has been found that if the twisted pair is intercepted at a distance close enough to the end user so that high bit rates (on the order of 25 Megabits per second (Mbps)) can be successfully delivered, then, depending upon the complexity of the loop transmission equipment, the loop must be shortened so as to have a length of at most approximately 500 to 3,000 feet.
This upper bound on loop length has led to the development of new access architectures, known in the art as Fiber-To-The-Cabinet (FTTCab), Fiber-To-The-Neighbourhood (FTTN), Fiber-To-The-Curb (FTTC) or Fiber-To-The-Building (FTTB), all generically referred to as Fiber-In-The-Loop (FITL). The FTTC architecture has been the method of choice when considering the delivery of broadband services to a residential area consisting of single-family dwellings.
Traditional FITL implementations provide a system in which a Host Digital Terminal (HDT) controls the FITL network and is located at, say, a central office. The HDT is connected on one side to core network resources and on another side (the "access side") to a series of dependent Optical Network Units (ONUs) via a fiber-based link in the form of a Passive Optical Network (PON), a Synchronous Optical Network (SONET) ring or a number of point-to-point links. Finally, the ONUs communicate bidirectional data with the individual end users along the final (short) stretches of copper.
At such short maximum loop lengths of only a few hundred feet, the number of subscribers that can be served by a single ONU is rather limited. Therefore, the ONU must be small, simple and inexpensive for the service provider to buy and install so that its initial cost can be borne by the revenues from the small number of subscribers that the ONU serves. Furthermore, having only a small group of subscribers served by any one ONU requires that a very large number of ONUs be deployed to create a ubiquitous access network. This demands that the ONUs, once installed, be individually very cheap to maintain while allowing for future changes in subscriber service requirements. Since the ONUs are placed deep in the "outside plant", any requirement which causes these ONUs to be visited, either for repair purposes or for provisioning different subscriber services (by changing line card functionality), will result in a system that is too costly to operate.
Conventional prior art FITL architectures, FTTC in particular, have adopted the approach of installing shelves or frames of equipment, including service-specific line cards, in a protective housing on the curbside. Such ONUs are large, complex and require regular visits, in order both to modify services by changing line card types and to repair the units, since more complex ONUs are more likely to fail. Hence, the cost of deploying an array of service-specific line cards is prohibitively high in terms of capital cost (complex electronics, large cabinets) and also in terms of operating costs due to the need to visit the ONU so as to implement a service type change by replacing the line card type. Furthermore, installing cabinet-mounted equipment is often complicated by the unavailability of acceptable locations in residential areas. This becomes more critical as the loop length is shortened and ONU size is reduced to the point where ONUs are installed within subdivisions and not at their edges.
An alternative prior art approach consists of replacing the service-specific line cards with (somewhat more expensive) service-independent line cards that can be configured in software. These are primarily based upon the use of wideband analog front-end loop drivers, oversampling codecs, bit-rate-reduction (decimator) blocks and digital filtering components, also known as Digital Signal Processor Application-Specific Integrated Circuits (DSP ASICs). This combination of functions allows the service-specific functions of the line card to be implemented in software, which can be downloaded to the ONU from the HDT, thereby eliminating the need to visit the ONU to change the service type delivered to a subscriber.
This solution, also referred to as Service-Adaptive Access (SAA), has been adopted by Nortel in the development of its S/DMS Access Node, which can be deployed in a FTTC or FTTCab configuration. The ONU, also called an RDT (Remote Digital Terminal), consists of an array of service-dependent line cards, or alternatively service-independent line cards based upon on-card DSP processing and each using a DSP dedicated to that card, or possibly (in order to control cost) a mix of both types of line cards, in addition to common equipment for multiplexing the digitized signals, a control processor and an optoelectronic transceiver. The number of different line card types can be reduced by replacing some or all of the standard POTS (Plain Old Telephone Service) cards with SAA line cards.
When data flows from the subscriber into the ONU, (known as the "upstream" path), the S/DMS Access Node samples the input analog signal arriving on the twisted pair and puts it into a standard digital format prior to transmission from the ONU to the HDT. In the opposite ("downstream") direction, the ONU converts, for example, .mu.-law-encoded digital voice data into an analog format for delivery to a user's home. Unfortunately, the deployment of such ONUs, each comprising a set of service-independent line cards, has several serious drawbacks in the context of a FITL system with deep fiber penetration:
1) Cost PA0 2) Size PA0 3) Complex software download PA0 4) Initial servicing PA0 5) Efficiency
The DSP-based line card has a larger power consumption, complexity and failure rate, which translates into significantly higher system cost; PA1 The size of the ONUs has increased, making it more difficult to install them in locations close to the end user; PA1 The ONU and access system at the HDT have to provide a high-integrity software download/verification path which requires a processor in each ONU for monitoring download integrity; PA1 The functionality of the individual line cards is such that the ONU must be visited each time a new subscriber is to be accommodated. The SAA cards do not allow "future-proofing", i.e. it is not possible to connect every loop to a line card (regardless of whether or not that loop was expected to go into service immediately) and then to remotely provision, or "initialize", that loop; PA1 The DSP is placed on the line card and as such is dedicated to a single loop. Furthermore, it has to be dimensioned for the most stringent expected processing demands that can be encountered in the loop. In combination, this leads to the number of high-performance DSPs deployed being equal to the number of lines served. Thus for many service types, including low-bandwidth POTS (the most common), each DSP may be operating at a fraction of its full capacity. However, this spare capacity cannot be shared across other loops, leading to an effective increase in power consumption and total system cost.
It is important to note that reducing the size of the ONU by reducing the number of DSP-based SAA line cards per ONU does little in the way of mitigating the above disadvantages. In fact, partitioning the equipment into smaller ONUs with lower line counts per ONU results in an increased overall complexity since the simplification achieved on a per-ONU basis is more than offset by the increased number of ONUs that have to be deployed. As the ONU line count falls, the overall complexity of the ONU population required to serve a particular area or group of subscribers rises and has deleterious consequences on the mean-time-between-failures (MTBF) of the ONU population, requiring a higher degree of maintenance activity. This translates into more frequent on-site visits ("truck rolls") by the repair crew and requires more travelling to the increased number of ONU sites.